1. Field of the Invention
Generally, the present disclosure relates to the manufacture of sophisticated semiconductor devices, and, more specifically, to various methods of forming bulk FinFET semiconductor devices so as to reduce punch through leakage currents.
2. Description of the Related Art
The fabrication of advanced integrated circuits, such as CPU's, storage devices, ASIC's (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements in a given chip area according to a specified circuit layout, wherein so-called metal oxide field effect transistors (MOSFETs or FETs) represent one important type of circuit element that substantially determines performance of the integrated circuits. A FET is a device that typically includes a source region, a drain region, a channel region that is positioned between the source region and the drain region, and a gate electrode positioned above the channel region. Current flow through the FET is controlled by controlling the voltage applied to the gate electrode. If a voltage that is less than the threshold voltage of the device is applied to the gate electrode, then there is no current flow through the device (ignoring undesirable leakage currents which are relatively small). However, when a voltage that is equal to or greater than the threshold voltage of the device is applied to the gate electrode, the channel region becomes conductive, and electrical current is permitted to flow between the source region and the drain region through the conductive channel region.
To improve the operating speed of FETs, and to increase the density of FETs on an integrated circuit device, device designers have greatly reduced the physical size of FETs over the years. More specifically, the channel length of FETs has been significantly decreased, which has resulted in improving the switching speed of FETs. However, decreasing the channel length of a FET also decreases the distance between the source region and the drain region. In some cases, this decrease in the separation between the source and the drain makes it difficult to efficiently inhibit the electrical potential of the channel from being adversely affected by the electrical potential of the drain. This is sometimes referred to as a so-called short channel effect, wherein the characteristic of the FET as an active switch is degraded.
In contrast to a planar FET, which has a planar structure, there are so-called 3D devices, such as an illustrative FinFET device, which is a 3-dimensional structure. More specifically, in a FinFET, a generally vertically positioned, fin-shaped active area is formed and a gate electrode encloses both sides and an upper surface of the fin-shaped active area to form a tri-gate structure so as to use a channel having a 3-dimensional structure instead of a planar structure. In some cases, an insulating cap layer, e.g., silicon nitride, is positioned at the top of the fin and the FinFET device only has a dual-gate structure. Unlike a planar FET, in a FinFET device, a channel is formed perpendicular to a surface of the semiconducting substrate so as to reduce the depletion width under the channel and thereby reduce so-called short channel effects. Also, in a FinFET, the junction capacitance at the drain region of the device is greatly reduced, which tends to reduce at least some short channel effects.
However, FinFET devices still exhibit some performance-limiting characteristics. One such characteristic that is detrimental to all forms of semiconductor devices, both FinFETs and planar FETs, is off-state leakage currents. Ideally, off-state leakage current is minimized to increase device performance. In the case of a FinFET device, it has been noted that the punch through leakage current densities vary along the vertical length of the fins with the current densities being much greater at the bottom of the fins proximate the local isolation regions of the device. See, e.g., Okano et al., “Process Integration Technology and Device Characteristics of CMOS FinFET on Bulk Silicon Substrate with sub-10 nm Fin Width and 20 nm Gate Length,” IEDM Technical Digest, pp. 721-724, 2005. Efforts have been made in the past in attempts to reduce such punch through leakage currents in FinFET devices. For example, in some cases, efforts have been made to form so-called punch through stoppers at the base of the fins by implanting ions into the bottom portion of the fins and/or oxidizing the bottom portion of the fins to effectively merge the local isolation regions together. However, as device dimensions continue to shrink, better and more efficient techniques are desired in an effort to eliminate or reduce detrimental leakage currents.
The present disclosure is directed to various methods of forming FinFET semiconductor devices that may solve or reduce one or more of the problems identified above.